Scintillation denotes the apparent temporal variation of the intensity of a remote radiation source. This effect is well known from star light twinkling. Physically scintillation may be defined as the variance of the intensity received at a detector. Instruments which measure scintillation are called scintillometers. Scintillometers generally consist of a transmitter and a receiver unit. The transmitter contains a source which emits radiation usually in the visible or infrared. At the receiver the intensity fluctuations are measured. The distance between transmitter and receiver typically is a few ten to a few hundred meters.
For such wavelengths and scales the scintillation magnitude, i.e. the variance of the intensity, is determined by the structure function constant C.sub.n.sup.2 and the inner scale l.sub.0 of the turbulent refractive index fluctuations of the air. C.sub.n.sup.2 is a measure of the amplitude of the refractive index fluctuations and l.sub.0 defines the smallest occuring turbulence eddies. The mathematics of this relationship is well known.
Scintillometers may be used to determine C.sub.n.sup.2 and l.sub.0. An important application of scintillometers is the determination of the turbulent fluxes of heat and momentum in the lowest atmosphere (V. Thiermann and H. Grassl, Boundary-Layer Meteorology 58 (1992), pp. 367). The advantages of using scintillation for turbulence measurements are the high accuracy, the averaging over the optical propagation path, and the purely optical, contact-free access.
In order to derive the two quantities C.sub.n.sup.2 and l.sub.0 from scintillation measurements, two independent informations must be available. So far the following methods have been proposed:
1. the simultaneous measurement of scintillation variances over two differently long propagation paths (P. M. Livingston, Applied Optics 11 (1972), pp. 684), PA0 2. the simultaneous measurement of scintillation variances of a coherent and an incoherent source (G. R. Ochs and R. J. Hill, Applied Optics 24 (1985), pp. 2430), PA0 3. the simultaneous measurement of scintillation variances and covariances at two different wavelengths (E. Azoulay, V. Thiermann, A. Jetter, A. Kohnle, Z. Azar, Journal of Physics D S21 (1988), pp. 21), PA0 4. the simultaneous measurement of scintillation variances and covariances at displaced detectors. Here the radiation is emitted from a single source (R. G. Frehlich, Applied Optics 27 (1988), pp. 2194).
Method 1 has the disadvantage that it compares scintillation over different paths where the turbulence is not necessarily the same. It thus requires spatial homogeneity of the turbulence field, otherwise errors will result. The methods 2 and 3 have the disadvantage that they need either two different wavelengths or a coherent and an incoherent source. The realization of such systems requires much technical effort.
Method 4 compares the covariance at at least two displaced detectors with the variance at a single detector. The detectors are illuminated by the same source. This method is technically simple. However, the spatial weighting functions for the variances and covariances are very different. The spatial weighting functions describe the contribution the different positions along the propagation path make to the measured variances or covariances. The weighting function of the variance is symmetric with its peak at the path center. The weighting function of the covariance is asymmetric with its peak close to the transmitter. If the variances and covariances are compared, turbulence statistics originating from different locations are compared and, as with method 1, inhomogeneities of the turbulence field may cause severe errors.